The present invention relates to a diagnostic apparatus for an exhaust gas clarification apparatus of an internal combustion engine using a plurality of catalyst converters arranged in series configuration, more specifically, to a diagnostic apparatus for an exhaust gas clarification apparatus of an internal combustion engine which diagnoses a status of the clarification apparatus by judging output signals from a plurality of air/fuel ratio sensors installed in the upper stream and the down stream of said plurality of catalyst converters.
An apparatus for clarifying the combustion exhaust gas in the internal combustion engine is generally composed of catalyst converters and an air/fuel ratio feedback control apparatus. A catalyst converter is mounted in the exhaust pipe for removing hazardous components in the exhaust gas, specifically including HC, NOx and CO. An air/fuel ratio feedback control apparatus is aimed to keep the air/fuel ratio constant in order to make the best use of function of catalyst converters, and controls the amount of fuel supplied to the engine in responsive to the air/fuel ratio which is obtained with an air/fuel ratio sensor or an oxygen sensor mounted on the upper stream of the catalyst converter in the exhaust gas pipe.
The three-way catalyst used in the above described catalyst converters has such a problem that the clarification performance is degraded even within the durable time predefined based on the expected aged-deterioration because the impurity materials stick to the catalyst part of the catalyst converters when supplying lead gasoline to the internal combustion engine as fuel.
As for the above described problem, in such a proposed system as disclosed in Japanese Laid-Open Patent 97852(1988), a secondary exhaust gas sensor for detecting oxygen density is so located in the down stream side of the catalyst converter in the exhaust system of the internal combustion engine as to judge the degradation of catalyst operation performance of the catalyst converter by referring to the number of altering output signals in the air/fuel ratio feedback control with the secondary exhaust gas sensor. In this proposed system, what is aimed is that there is no difference between the oxygen density in the down stream side of the catalyst and that in the upper stream of the catalyst due to the decease in the oxygen absorption power of the catalyst when the catalyst operation performance is degraded, and that this indifference is used for judging indirectly the degradation of the catalyst.
As for the prior art related to the diagnosis of plurality of catalysts arranged in series in the exhaust gas system, an example is disclosed in Japanese Laid-Open Patent 26032(19993). In this prior art system, a secondary (second) exhaust gas sensor is installed in the in-between part between the upper stream catalyst converter and the down stream catalyst converter in the exhaust gas system, and a tertiary (third) exhaust gas sensor is installed in the down stream part of the down stream catalyst. In this system, under the air/fuel ratio feedback control based on the primary (first) exhaust gas sensor, the degradation of the upper stream catalyst is judged by referring to the number of inversions in the second exhaust gas sensor. In addition, under the air/fuel ratio feedback control based on the second exhaust gas sensor, the degradation of the upper stream catalyst is judged by referring to the number of inversions in the third exhaust gas sensor.
In such a prior art as having a plurality of catalysts arranged in series in the exhaust gas system, the following problem arises when simply applying a configuration including an exhaust gas sensor located in the down stream side of the down stream catalyst converter and judging the degradation of the catalyst based on the changes in the output signal from this sensor.
As the components and their individual concentration of the combustion exhaust gas supplied to the upper stream catalyst converter is different from those of the gas supplied to the down stream catalyst converter, the degree of degradation varies with the individual catalyst converters, which leads to the difficulty in keeping stable reliability.
Regarding to the above problem, what may be considered is such a solution that an exhaust gas sensor for judging the degradation of the upper stream catalyst is mounted in the in-between part between the upper stream catalyst converter and the down stream catalyst converter in the exhaust gas system. However, as larger amount of oxygen in the exhaust gas is absorbed in the upper stream and down stream catalyst converters compared with the case of using a single unit of catalyst converter, the partial pressure of oxygen in the combustion exhaust gas in the down stream side of the down stream catalyst converter does not change extremely when the air/fuel ratio feedback control is based on the output signal from the exhaust gas sensor located in the upper stream side of the upper stream catalyst converter. Under this circumstance, any explicit change in the output signal from the exhaust gas sensor located in the down stream side of the down stream catalyst converter can not be seen other than the case that the catalyst performance is extremely degraded, which leads to the problem that the accuracy in degradation judgment of the down stream catalyst converter may be reduced.
Thus, in the prior art, as an attempt is made to solve above described problems by applying an air/fuel ratio feedback control based on the output signal from the exhaust gas sensor installed in the in-between part of the upper stream and down stream catalysts. However, as engine control parameters are forced to be changed for diagnostic operations for the degradation of catalyst, another new problem including by-effect to exhaust system characteristic and performance should be considered.